1. Field of the Invention
The present invention relates generally to intraocular lenses, and more specifically to intraocular lenses for providing accommodative vision.
2. Description of the Related Art
A human eye includes a transparent crystalline biconvex lens which can focus light from objects over a wide range of distances on the retina. The lens allows the eye to focus on objects at various distances by changing its shape thereby changing its focal length. The ability of the lens to change its shape to adjust focal length is known as accommodation. FIG. 1 is a schematic illustration of a cross-section of the natural crystalline lens 100. The lens 100 is connected to the ciliary body at the periphery of the lens generally in the equatorial plane. The lens 100 has an optical axis A-A′ which extends from the anterior pole through the posterior pole. The lens 100 has three main parts: the lens capsule 105, the lens epithelium 110, and the lens fibers 115. The lens capsule 105 forms the outermost layer of the lens 100 and the lens fibers 115 form the bulk of the interior of the lens 100 with the cells of the lens epithelium 110, located between the lens capsule 105 and the outermost layer of lens fibers 115 on the anterior side of the lens.
The lens capsule 105 is an elastic, transparent membrane that completely surrounds the lens. The lens epithelium 110 is located in the anterior portion of the lens between the lens capsule 105 and the lens fibers 115 and is responsible for regulating of the homeostatic functions of the lens. The cells of the lens epithelium 110 also serve as the progenitors for new lens fibers.
The lens fibers 115 are long, thin, transparent cells that are firmly packed within the lens capsule 105. The lens fibers 115 stretch lengthwise from the anterior pole to the posterior pole with the middle of each fiber located on the equator. The lens fibers 115 are arranged in concentric layers rather like the layers of an onion in a cross-sectional view of the lens along a plane perpendicular to the equatorial plane as illustrated in FIG. 1. The lens fibers appear as a honeycomb a cross-sectional view of the lens along the equatorial plane (not shown).
The refractive index of the lens fibers 115 increases progressively from the outermost lens fibers 115 that are adjacent to the lens capsule having a refractive index of approximately 1.38 to the inner most lens fibers having a refractive index of approximately 1.41. The refractive index gradient is advantageous in enhancing the optical power of the lens.
A human eye can suffer diseases that impair a patient's vision. For instance, a cataract may increase the opacity of the lens, causing blindness. To restore the patient's vision, the opaque lens may be surgically removed and replaced with an artificial intraocular lens, or IOL. An artificial IOL may also be implanted to treat presbyopia or for other elective ocular surgical procedures.
The artificial IOL can be an accommodating IOL, which can adjust its axial position and/or shape to vary the optical power within a range in response to muscle action in the eye. As a result, the patient can focus on objects in a range of distances from the eye, rather than at one or more discrete distances.